Filter assemblies are used in industrial facilitates to selectively remove material from process streams. Filter assemblies are used to both remove undesirable contaminates from process streams and to extract desirable materials (both hereafter termed filtrates) out of the process streams. Two such filter assemblies are disclosed in the Applicants' Assignees' U.S. Pat. No. 5,198,111, FILTER WITH RECIPROCATING CLEANING UNIT, issued Mar. 30, 1993 and U.S. Pat. No. 5,527,462, FILTER WITH AXIALLY MOVABLE WIPER, issued Jun. 18, 1996, both of which are incorporated herein by reference.
Generally, each of the above-referenced filter assemblies includes an elongated, cylindrically shaped housing. Internal to the housing there is a sleeve shaped filter element. The process stream to be filtered is introduced into the center of the filter element, usually through the top of the housing. The process stream is filtered as it passes from the inside to the outside of the filter member. The filtered process stream exits the filter assembly through an opening in the housing.
As the process stream passes through a filter assembly, the removed filtrate builds up on the inner wall of the filter element. If allowed to remain in place, the filtrate will, over time, first impede and then possibly block fluid flow through the filter assembly. Therefore, it is a common practice to further provide the filter assembly with a cleaning head. This cleaning head moves longitudinally up and down through the filter element to scrape off the filtrate that adheres to the element. Typically, the removed filtrate is allowed to flow through the bottom of the filter element and out through an opening in the bottom of the housing.
U.S. Pat. No. 5,198,111 discloses a filter assembly wherein the cleaning head is mounted to a threaded shaft that extends longitudinally axially through the housing and filter element. A motor mounted to the housing rotates the shaft so as to cause the longitudinal displacement of the cleaning head. U.S. Pat. No. 5,527,462 discloses a filter assembly wherein the cleaning head is mounted to the end of a piston rod that extends down from the top of the housing into the filter element. The opposed end of the piston rod is disposed in a cylinder attached to the top of the housing. A fluid, such as compressed air, is applied to the cylinder in order to cause the extension/retraction of the piston rod and cleaning head through the filter element.
One disadvantage associated with providing a filter assembly with a screw type cleaning head actuator is that the actuator has components that come into contact with the process stream. Steps must be taken to ensure that the upline components that drive the moving components are not exposed to the process stream. For example, it is necessary to ensure that there is a liquid-tight seal around the opening through which the screw shaft enters the housing. A similar seal must be provided around the opening where the piston of the above-described linear actuator enters the housing. It should be understood that fluid typically enters the housing under a significant amount of pressure, for example 150 psi. In order to successfully perform the filter process, it may be necessary to repetitively actuate the cleaning head. Therefore, given that the seal around the actuator is subjected to significant pressure and the actuator itself is repeatedly actuated, this seal is subjected to significant wear. This wear requires that an appreciable amount of effort be spent verifying that the seal is maintaining its integrity, and when that integrity shows signs of failing, replacing the seal.